THE SEQUENCE OF THE 5'-END OF THE U8 SMALL NUCLEOLAR RNA IS CRITICAL FOR 5.8S AND 28S RIBOSOMAL-RNA MATURATION

Ribosome biogenesis in eucaryotes involves many small nucleolar ribonu cleoprotein particles (snoRNP), a few of which are essential for proce ssing pre-rRNA. Previously, U8 snoRNP was shown to play a critical rol e in pre-rRNA processing, being essential for accumulation of mature 2 8S and 5.8S rRNAs. Here, evidence which identifies a functional site o f interaction on the U8 RNA is presented, RNAs with mutations, inserti ons, or deletions within the 5'-most 15 nucleotides of U8 do not funct ion in pre-rRNa processing, In vivo competitions in Xenopus oocytes wi th 2'O-methyl oligoribonucleotides have confirmed this region as a fun ctional site of a base pairing interaction, Cross-species hybrid molec ules of US RNA show that this region of the US snoRNP is necessary for processing of pre-rRNA but not sufficient to direct efficient cleavag e of the pre-rRNA substrate; the structure or proteins comprising, or recruited by, the U8 snoRNP modulate the efficiency of cleavage, Intri guingly, these 15 nucleotides have the potential to base pair with the 5' end of 28S rRNA in a region where, in the mature ribosome, the 5' end of 28S interacts with the 3' end of 5.8S, The 28S-5.8S interaction is evolutionarily conserved and critical for pre-rRNA processing in X enopus laevis. Taken together these data strongly suggest that the 5' end of US RNA has the potential to bind pre-rRNA and in so doing, mag regulate or alter the pre-rRNA folding pathway. The rest of the US par ticle mag then facilitate cleavage or recruitment of other factors whi ch are essential for pre-rRNA processing.